The present invention relates in general to apparatus and methods for detecting the coagulation of blood, and in particular to microprocessor-controlled apparatus and method for detecting changes in a magnetic field adjacent to a zone containing a blood sample, which changes indicate that the blood has coagulated.
The formation of clots within the circulatory system has been known to result in serious, if not fatal, consequences, particularly when the clot lodges within the heart or the brain. To prevent clots from occurring, coagulation inhibiting agents are normally administered to a patient. As a prerequisite for administering the coagulation inhibiting agents, the level of inhibiting agents within the patient's circulatory system must be known. The time required for clot formation within a standard laboratory test tube has been interpreted as an indication of the level of such inhibiting agents within a patient's circulatory system.
A system and a method for automatically measuring clotting time is disclosed in U.S. Pat. No. 3,695,842 entitled "METHOD AND SYSTEM FOR ANALYZING A LIQUID", issued to Michael D. Mintz on Oct. 30, 1972 and assigned to International Technidyne Corporation, the assignee of the present invention. A sample of blood is placed in a test tube and a permanent magnet is immersed in the blood sample. A magnetic reed switch, which is normally open, is positioned directly below the magnet. Flux lines, provided by the magnet, pass through the reed switch, causing it to close. Then, a relative rotational motion is produced between the test tube and the magnet to agitate the blood, during which time the magnet remains positioned over the reed switch. When the blood coagulates, the resulting fibrous strands of clotted sample causes the magnet to move conjointly with the test tube. Thus, the magnet is displaced from the reed switch. This displacement causes a reduction in the density of the magnetic flux lines passing through the reed switch (i.e., weakens the magnetic field). As a result, the reed switch opens and a signal is generated, indicating the occurrence of the coagulation of blood.
An improved system for measuring clotting time is disclosed in U.S. Pat. No. 3,836,333 entitled "SYSTEM FOR TIMING THE COAGULATION OF BLOOD" issued to Michael D. Mintz on 30 Oct. 1972 and assigned to International Technidyne Corporation. An electromagnetic bias coil, which is wound around the reed switch, provides steady-state magnetic flux lines that supplement the flux lines provided by the permanent magnet. When the density of the flux lines passing through the reed switch decreases as a result of the magnet being displaced, the reed switch opens. The bias coil also provides a magnetic pulse, which forces the reed switch to a closed state. This system is manufactured under the trademark HEMOCHRON by International Technidyne Corporation at Edison, N.J.
The precision with which the system detects the coagulation of blood is dependant upon the ability of the reed switch to respond to changes in density of the magnetic flux lines. To ensure that the reed switch opens and closes in the presence of the correct flux densities, the reed switch must be manufactured with a great deal of precision. When tested, any reed switch that does not operate as specified must be eliminated. This results in additional expense as high precision switches are inherently more costly.
One problem encountered in using a reed switch for magnetic field measurement relates to the process of magnetic hysteresis. The effect of hysteresis in a reed switch is to require a greater magnetic flux density to initially close the reeds than that required to simply maintain the reeds in a closed condition. In the system described above, the difference between the magnetic field required to close the reeds and the magnetic field at which the reeds just reopened must be less than the difference in magnetic field passing through the reed switch when the magnet is directly above the reed switch and when the magnet has been displaced relative to the reed switch.
A second problem with reed switches relates to magnetic storage or magnetization. The reeds of the switch are made of a ferromagnetic material. During operation, when magnetic flux lines pass through the reed switch, the reeds store energy at a slow rate and, therefore, become magnetized. As a result of the stored magnetic energy, the reed switch becomes a time-dependant storage device, which may fail to open or close precisely at predetermined external flux level. Thus, a reed switch employed in the system described above may not switch states precisely when the blood clots.
Therefore, it is an object of the present invention to eliminate the need for a high-precision reed switch in a system for detecting the coagulation of blood.
It is a further object of the present invention to eliminate the effects of magnetic hysteresis and magnetic storage in the reed switch in such a system.
It is still a further object of the present invention to provide a technique for calibrating a reed switch in such a system.